1. Field of the Invention
The present invention relates to a thermosetting resin composition which exhibits excellent curing property and storage stability and advantageously used in the electric and electronic fields, an epoxy resin molding material using the composition and a semiconductor device using the material.
2. Description of Related Art
For electric and electronic materials and, in particular, for sealing materials for semiconductors, improvement in the curing rate has been required to increase production efficiency and improvement in storage stability has been required to improve handling during distribution and storage.
In epoxy resins used in the field of electric and electronic materials, amines, imidazole-based compounds, heterocyclic compounds having nitrogen such as diazabicycloundecene, quaternary ammonium compounds, phosphonium compounds, arsonium compounds and the like various compounds have been used as the curing catalyst.
When amines and, in particular, imidazole-based compounds are used for a sealing material for semiconductors, these compounds cause corrosion of internal wiring under a condition of a high temperature and a high humidity, i.e., these compounds tend to show poor reliability under humid conditions, although these compounds exhibit an excellent curing property. Therefore, the use of the above compounds is not desirable in the field of the electric and electronic materials. Phosphorus-based compounds such as phosphonium compounds are widely used in this field.
Many of the widely used curing catalysts exhibit the effect of accelerating curing even at relatively low temperatures such as the ordinary temperature. This effect causes deterioration in the quality of the resin composition. For example, viscosity increases and fluidity decreases during production and storage and the curing property fluctuates.
To overcome the above problem, so-called latent curing accelerators which suppress the change in viscosity and fluidity at low temperatures with time and exhibit the effect of accelerating curing reaction only under heating during molding have been studied widely. For this purpose, studies for exhibition of the latent property by protecting the active site of a curing accelerator with an ion pair have been conducted. In Japanese Patent Application. Laid-Open No. Heisei 8(1996)-41290, a latent curing accelerator having a salt structure formed with an acid selected from various organic acids and phosphonium ion, is disclosed. However, since this phosphonium salt does not have a specific higher order molecular structure and the ion pair is relatively easily susceptible to the influence of the outside environment, a problem arises in that storage stability decreases when this phosphonium salt is applied to recent sealing materials for semiconductors which uses phenol resin curing agents exhibiting greater mobility of molecules such as low molecular weight epoxy resins and phenol aralkyl resins. In Japanese Patent Application Laid-Open No. 2001-98053, a curing accelerator which is a molecular association compound of a tetrasubstituted phosphonium compound, a compound having at least two phenolic hydroxyl groups in one molecule and a conjugate base of the compound having at least two phenolic hydroxyl groups in one molecule, is disclosed. This molecular association compound corresponds to molecular compound (C) represented by general formula (1) or (2) described in the present invention in which m represents 1, However, this molecular association compound has a problem in that the active site in the reaction is strongly protected by the steric hindrance due to the higher order molecular structure and the curing property decreases although the storage stability is excellent.
The present invention has an object of providing a thermosetting resin composition which exhibits excellent curing property and storage stability and is advantageously used in the field of electric and electronic materials, an epoxy resin molding material using the composition and a semiconductor device exhibiting excellent reliability under humid conditions.
The present invention provides
[1] A thermosetting resin composition which comprises as essential components thereof (A) a compound having at least two epoxy groups in one molecule, (B) a compound having at least two phenolic hydroxyl groups in one molecule and (C) a molecular compound represented by any one of general formula (1) and general formula (2); general formula (1) being: 
wherein P represents phosphorus atom, R1, R2, R3 and R4 each represent a substituted or unsubstituted aromatic group or an alkyl group, A1 represents a divalent aromatic group, B1 represents a single bond, a divalent group selected from ether groups, sulfone groups, sulfide groups and carbonyl groups or a divalent organic group having 1 to 13 carbon atoms and m represents a number in a range of 0xe2x89xa6mxe2x89xa60.75; and general formula (2) being: 
wherein P represents phosphorus atom, R1, R2, R3 and R4 each represent a substituted or unsubstituted aromatic group or an alkyl group, A2 represents a divalent aromatic group and m represents a number in a range of 0xe2x89xa6mxe2x89xa60.75;
[2] An epoxy resin molding material which comprises as essential components thereof (A) a compound having at least two epoxy groups in one molecule, (B) a compound having at least two phenolic hydroxyl groups in one molecule, (C) a molecular compound represented by any one of general formula (1) and general formula (2) and (D) an inorganic filler; general formula (1) being: 
wherein P represents phosphorus atom, R1, R2, R3 and R4 each represent a substituted or unsubstituted aromatic group or an alkyl group, A1 represents a divalent aromatic group, B1 represents a single bond, a divalent group selected from ether groups, sulfone groups, sulfide groups and carbonyl groups or a divalent organic group having 1 to 13 carbon atoms and m represents a number in a range of 0xe2x89xa6mxe2x89xa60.75; and general formula (2) being: 
wherein P represents phosphorus atom, R1, R2, R3 and R4 each represent a substituted or unsubstituted aromatic group or an alkyl group, A2 represents a divalent aromatic group and m represents a number in a range of 0xe2x89xa6mxe2x89xa60.75;
[3] A material described in [2], wherein, in molecular compound (C) represented by any one of general formula (1) and general formula (2), the number represented by m is 0 or 0.5;
[4] A material described in [3], wherein the phenol component in molecular compound (C) represented by general formula (1) is bis(4-hydroxyphenyl) sulfone, 2,2-bis(4-hydroxyphenyl)-1,1,1,3,3,3-hexafluoro-propane or bis(4-hydroxyphenyl) ether; and
[5] A semiconductor device which comprises a semiconductor element sealed with an epoxy resin molding material described in any one of [2], [3] and [4].
An epoxy resin molding material which exhibits remarkably excellent curing property and storage stability can be obtained in accordance with the present invention described above.
Compound (A) having at least two epoxy groups in one molecule which is used in the present invention is not particularly limited as long as the compound has at least two epoxy groups in one molecule. Examples of compound (A) include epoxy resins and epoxy compounds obtained by reacting epichlorohydrin with hydroxyl group in phenols, phenol resins and naphthols such as epoxy resins of the biphenyl type, epoxy resins of the novolak type and epoxy resins of the naphthalene type; epoxy resins obtained by epoxidation of olefins with peracids such as alicyclic epoxy resins; and compounds obtained by epoxidation of dihydroxybenzenes such as hydroquinone with epichlorohydrin.
Compound (B) having at least two phenolic hydroxyl groups in one molecules works as the curing agent for the compound (A) having at least two epoxy groups in one molecule. Examples of compound (B) include phenol novolak resin, cresol novolak resins, novolak resins modified with alkyl groups (including resins obtained by co-condensation of the double bond of cycloalkenes with phenols in accordance with the Friedel-Crafts reaction), phenol aralkyl resins and resins obtained by co-condensation of naphthols and phenols with compounds having carbonyl group. However, compound (B) is not limited to the compounds described above and any compound in which hydrogen atoms bonded to aromatic rings are substituted with at least two hydroxyl groups in one molecule can be used.
Molecular compound (C) which works as the curing accelerator in the present invention is represented by any one of general formula (1) and general formula (2) and is a molecular association compound of tetrasubstituted phosphonium and a phenol compound. This compound comprises a combination of one tetrasubstituted phosphonium cation, at least one and less than three phenolic hydroxyl groups and one phenoxide anion. It is considered that the positive charge of the tetrasubstituted phosphonium cation is surrounded with at least one and less than three phenolic hydroxyl groups and one phenoxide anion and a stable structure is formed.
As the phosphonium ion which can have the above structure, a tetrasubstituted phosphonium ion having substituted or unsubstituted aryl groups and alkyl groups as the substituents is preferable due to stability against heat and hydrolysis. Examples of the phosphonium ion include tetratolylphosphonium ions such as tetraphenylphosphonium ion and tetratolylphosphonium ion; triarylmonoalkylphosphonium ions synthesized from triarylphosphines and alkyl halides such as triphenylmethylphosphonium ion; and tetraalkylphosphonium ions such as tetrabutylphosphonium ion.
Molecular compound (C) used in the present invention has a structure of the phosphonium-phenoxide salt type as described above. This structure is different from the structure of the conventional compounds of the phosphonium-organic acid anion salt type in that, in molecular compound (C), a higher order structure formed with the hydrogen bonds of protons of the phenolic hydroxyl groups surrounds the ionic bond. In the conventional salt, the reactivity is controlled by the strength of the ionic bond alone. In contrast, in molecular compound (C), the ion pair of the active site of the reaction is surrounded by the higher order structure and protected at the room temperature. In the stage of the actual molding, the higher order structure is decomposed and the active site of the reaction is exposed to exhibit the reactivity. Thus, the so-called latent property can be provided.
Examples of the phenol compound which can, in general, form a molecular compound with the tetrasubstituted phosphonium ion include bisphenols such as bisphenol A (2,2-bis(4-hydroxyphenyl)propane), bisphenol F (4,4xe2x80x2-methylenebisphenol, 2,4xe2x80x2-methylenebisphenol and 2,2-methylenebisphenol), bis(4-hydroxyphenyl) sulfone (bisphenol S), bisphenol E (4,4xe2x80x2-ethybdenebisphenol), bisphenol fluorene (4,4xe2x80x2-(9H-fluoren-9-ylidene)bisphenol), 4,4xe2x80x2-methylidenebis(2,6-dimethylphenol) and bis(4-hydroxyphenyl)methanone; biphenols such as 4,4xe2x80x2-biphenol, 2,2xe2x80x2-biphenol, 3,3xe2x80x2,5,5xe2x80x2-tetramethylbiphenol and 2,2-bis(4-hydroxyphenyl)-1,1,1,3,3,3-hexafluoropropane; hydroquinone; resorcinol; catechol; bis(4-hydroxyphenyl) ether; 2,6-dihydroxynaphthalene; 1,4-dihydroxynaphthalene, 2,3-dihydroxynaphthalene; 1,6-dihydroxynaphthalene; 1,1xe2x80x2-bi-2-naphthol; and 1,4-dihydroxyanthraquinone. These compounds can form coordinated structures represented by general formula (1) or general formula (2) in which the number represented by m can be in the range of 0 to 2 depending on the relative amounts of the tetrasubstituted phosphonium ion and the phenol compound and the structure of the phenol compound. In the present invention, phenol compounds are selected among the above compounds so that the number represented by m is in the range of 0xe2x89xa6mxe2x89xa60.75 in the formed coordinated structure represented by general formula (1) or general formula (2). The structures of the tetraphenylphosphonium portion and the phenol portion can be confirmed by the measurement of 1H-NMR and the mass spectrum. The value of m in general formula (1) or general formula (2) can be calculated from the integral intensities of the phosphonium portion and the phenol portion obtained by the measurement of 1H-NMR of the molecular compound in accordance with the following equation:
m=(F1/F0)/(P1P0)xe2x88x921
wherein P0 and F0 each represent the number of proton in one molecule of the phosphonium cation and P1 and F1 represent the integral values of the phosphonium cation portion and the total phenol portion in the anion side, respectively, which are obtained by the measurement.
The value of m in general formula (1) or general formula (2) representing the molecular compound is decided depending on the stability of phenoxy ion, easiness of hydroxyl group for forming the hydrogen bond, steric hindrance and rigidify of the phenol compound and other factors. It is considered from the steireochemical consideration that m can have discrete values such as 0.0, 0.25, 0.33, 0.5, 0.67, 0.75, 1.0, 1.25, 1.33, 1.5 and so on. Examples of the phenol compound which is preferable for forming the structure represented by general formula (1) or general formula (2) in which the number represented by m is in the range of 0xe2x89xa6mxe2x89xa60.75 include bisphenol S, 2,2-bis(4-hydroxyphenyl)-1,1,1,3,3,3-hexafluoropropane, bis(4-hydroxy-3-methylphenyl) sulfone and bis(4-hydroxyphenyl) ether. There is the possibility that the formed structure is different depending on the relative amounts of the phosphonium compound and the phenol compound used in the synthesis of the molecular compound. When m in general formula (1) or general formula (2) represents a number in the range of 0xe2x89xa6mxe2x89xa60.75, the higher order structure is easily decomposed and a catalyst exhibiting a greater activity of curing the epoxy resin can be obtained. On the other hand, when m in general formula (1) or general formula (2) represents a number of 1 or greater, the protection of the active site of the reaction is strong due to the steric hindrance of the phenol compound and the reactivity at the molding temperature decreases.
Molecular compound (C) can be synthesized in accordance with any of the following processes. In one of the processes, the above phenol compound and a base for helping ultimate removal of a hydrogen halide which is, for example, an alkali metal hydroxide such as sodium hydroxide and potassium hydroxide or an organic base such as pyridine and triethylamine are dissolved into a solvent such as an alcohol, a halide of the above tetrasubstituted phosphonium ion dissolved in a suitable solvent is added, the reaction is allowed to proceed and the object compound is separated as the solid component in accordance with a separating operation such as recrystallization and reprecipitation in the last step. In another process, a tetrasubstituted phosphonium tetrasubstituted borate and the phenol compound are reacted with generation of heat and further reacted in a solvent such as an alcohol under heating.
It is preferable due to the excellent balance of the curing property, the storage property and other properties that the amount of molecular compound (C) used as the curing accelerator in the present invention is in the range of about 0.5 to 20 parts by weight per 100 parts by weight of the total of the amount of compound (A) having at least two epoxy groups in one molecule and the amount of compound (B) having at least two phenolic hydroxyl groups in one molecule which works as the curing agent. As for the relative amounts of compound (A) having at least two epoxy groups in one molecule and compound (B) having at least two phenolic hydroxyl groups in one molecule, it is preferable that the total of the amount by mole of the phenolic hydroxyl group in compound (B) having at least two phenolic groups in one molecule and the amount by mole of the phenolic hydroxyl group in molecular compound (C) is in the range of about 0.5 to 2 moles and preferably in the range of about 0.8 to 1.2 moles per 1 mole of epoxy group in compound (A) having at least two epoxy groups in one molecule since the curing property, heat resistance and electric properties are improved.
The type of the inorganic filler of component (D) used in the present invention is not particularly limited and inorganic fillers conventionally used for sealing materials can be used. Examples of the inorganic filler include fused and crushed silica powder, fused spherical silica powder, crystalline silica powder, secondary aggregated silica powder, alumina, titanium white, aluminum hydroxide, talc, clay and glass fiber. Fused spherical silica powder is preferable. It is preferable that the shape is close to the perfect sphere as much as possible. The spherical silica can be used in an increased amount by using powders having different particle sizes.
It is preferable that the amount of the inorganic filler is in the range of 200 to 2,400 parts by weight per 100 parts by weight of the total of the amount of compound (A) having at least two epoxy groups in one molecule and the amount of compound (B) having at least two phenolic hydroxyl groups in one molecule. When the amount is less than 200 parts by weight, there is the possibility that the effect of reinforcement with the filler is not sufficiently exhibited. When the amount exceeds 2,400 parts by weight, there is the possibility that fluidity of the molding material decreases and defective filling takes place during the molding. Therefore, such amounts are not preferable. It is more preferable that the amount of the inorganic filler is in the range of 250 to 1,400 parts by weight per 100 parts by weight of the total of the amounts by weight of compounds (A) and (B) since the degree of moisture absorption of the product obtained by curing the molding material decreases to prevent formation of cracks in solder and viscosity of the molding material in the melted condition decreases to eliminate the possibility of deforming metal wiring inside the semiconductor device. It is preferable that the inorganic filler is mixed well in advance.
The epoxy resin molding material of the present invention may further comprise various additives in combination with components (A) to (D), where necessary. Examples of the additive include coupling agents such as xcex3-glycidoxypropyltrimethoxysilane, coloring agents such as carbon black, flame retardants such as brominated epoxy resins, antimony oxide and phosphorus compounds, low stress components such as silicone oils and silicone rubbers, mold release agents such as natural wax, synthetic wax, higher fatty acids, metal salts of higher fatty acids and paraffins and antioxidants. In the present invention, catalysts such as triphenyl-phosphine and 1,8-diazabicyclo(5,4,0)undecene-7,2-methyl-imidazole may be used in combination without any problems as long as the function of molecular compound (C) as the curing accelerator is not adversely affected.
The epoxy resin molding material of the present invention can be obtained by mixing components (A) to (D) and other additives at the room temperature by a mixer, followed by kneading by a mixing machine such as rolls and an extruder, cooling and pulverizing.
In the production of the semiconductor device by sealing electronic parts such as semiconductors using the epoxy resin molding material of the present invention, the molding and curing can be achieved in accordance with the transfer molding, the compression molding or the injection molding.
The semiconductor device prepared by sealing with the cured product of the epoxy resin molding composition of the present invention is included in the technical range of the present invention and exhibits excellent resistance to moisture.
To summarize the advantages obtained by the present invention, the thermosetting resin composition and the epoxy resin molding material of the present invention exhibit excellent curing property and storage stability and the semiconductor device obtained by sealing a semiconductor element with the epoxy resin molding material exhibits excellent reliability;under humid conditions. Therefore, the composition, the material and the device are used industrially advantageously.